1. Field of the Invention
The present invention relates to a combined laser source which optically combines laser beams emitted from a plurality of semiconductor lasers by using an optical condensing system.
2. Description of the Related Art
Conventionally, the combined laser sources which condense and optically combine a plurality of laser beams emitted from a plurality of laser-light sources by using an optical condensing system are known as high-power laser-light sources which can be used in exposure systems and the like. FIG. 25 is a plan view of an example of such a combined laser source. In the combined laser source illustrated in FIG. 25, the laser beams B1 to B7 emitted from the seven semiconductor-laser chips L1 to L7, which are aligned along a line and fixed on a heat block 10, are respectively collimated by the seven collimator lenses 11 to 17 and are condensed by the condensing lens 21 so that the collimated and condensed laser beams are optically combined on the entrance end face of the core 30a of the optical fiber 30, as disclosed in U.S. Pat. No. 6,718,088).
In addition, as disclosed in Japanese Unexamined Patent Publication No. 2004-077779, a combined laser source in which a plurality of semiconductor-laser chips are arranged along a circle on a heat block is known, where laser beams emitted from the semiconductor-laser chips are collimated by collimator lenses and are condensed by a condensing lens so that the collimated and condensed laser beams are optically combined on a light-entrance end face of a core of an optical fiber.
When the combined laser sources as disclosed in U.S. Pat. No. 6,718,088 and Japanese Unexamined Patent Publication No. 2004-077779 are used in image exposure systems, the output intensities of the combined laser sources are required to be further increased in order to reduce the exposure time.
The output intensity of a combined laser source corresponds to the amount of light per unit area. The amount of light per unit area can be increased by minimizing the core diameter of the optical fiber which the condensed laser beams enter, or increasing the number of the laser beams which are optically combined (i.e., the number of the semiconductor lasers).
It may be considered that the numerical aperture (NA) of the optical fiber should be increased in order to increase the number of the laser beams which are optically combined. However, when the numerical aperture (NA) is increased, the focal depth decreases. Nevertheless, there is a demand to increase the focal depth for relaxing tolerance in assembly of the image exposure system in which the combined laser source is to be installed. Therefore, it is impossible to increase the numerical aperture (NA) of the optical fiber.
It is possible to increase the number of the laser beams to be optically combined, without increase in the numerical aperture (NA), by increasing the focal length of the condensing lens. However, when the focal length increases, the optical magnification power increases. Since the images of the emission points of the semiconductor lasers are magnified by the magnification power, and the magnified images of the emission points are formed on the light-entrance end face of the optical fiber, the sizes of the images of the emission points at the light-condensing position are increased by the magnification power. Nevertheless, the increase in the sizes of the images of the emission points at the light-condensing position is incompatible with the reduction in the core diameter of the optical fiber. Therefore, it is unpreferable to increase the focal length of the condensing lens.
In particular, the magnification power is an unignorable factor in the case where the semiconductor lasers have broad emission areas, although the emission points of the single-mode semiconductor lasers are small.
In addition, when the magnification power increases, higher precision is required in the position adjustment of the semiconductor lasers and the collimator lenses. When the position adjustment is inaccurate, and the laser beams deviate from the core of the optical fiber, loss occurs in the amount of light, so that the output intensity cannot be increased. Further, since the operation for the position adjustment is required to be repeated the number of times corresponding to the number of the semiconductor lasers, the requirement of the high precision in the position adjustment of the semiconductor lasers and the collimator lenses is a serious cost-increasing factor.
In order to increase the output intensity of the combined laser sources in which semiconductor lasers are arranged as mentioned before, it is effective to closely arrange the semiconductor lasers so that the gaps between the semiconductor lasers and between the collimator lenses are as small as possible. However, if the semiconductor lasers and the collimator lenses are too closely arranged, it is impossible to allow sufficient space to mount the semiconductor lasers and the collimator lenses. Therefore, the close arrangement of the semiconductor lasers and the collimator lenses imposes constraints on the constituents per se and adjustment of the constituents. Thus, the close arrangement of the semiconductor lasers and the collimator lenses is disadvantageous in terms of the cost of the components, devices for adjustment, and the man-hours needed for adjustment, and increases the total cost.
In a method which has been proposed for reduction of the man-hours needed for adjustment, a plurality of semiconductor lasers are integrally formed into a semiconductor-laser array, a plurality of collimator lenses are integrally formed into a collimator-lens array, and the positions of the semiconductor-laser array and the collimator-lens array are adjusted when the semiconductor-laser array and the collimator-lens array are mounted. However, the operation of mounting the chips increases man-hours needed for quality assurance. In addition, since the semiconductor-laser array and the collimator-lens array are custom-built, the cost of a high-precision mounting device adapted for the custom-built components is high, so that it is a heavy burden to recover the cost of the mounting device. Therefore, it is almost impossible to reduce the cost by using the above method.
Further, in the conventional combined laser source in which a number of semiconductor-laser chips area arranged on a heat block, the entire combined laser source is required to be sealed, and the yield rate is low, since the entire combined laser source becomes a defective even when only one of the chips or the lenses is defective. Thus, the total cost of the conventional combined laser source is high.
If the conventional semiconductor lasers mounted in a sealed package are used instead of the separate semiconductor-laser chips, the above-mentioned problems of the necessity of sealing or the low yield rate do not occur. However, the sizes of the sealed packages of the conventional semiconductor lasers are large, so that the gaps between the light-emission points of the semiconductor lasers increase, and the amount of light per unit area in a plane in the condensing lens decreases. In addition, the numerical aperture (NA) of the optical fiber is required to be great in order to make the widely spaced laser beams condensed and inputted into the optical fiber. This requirement is contrary to the aforementioned requirement for small numerical aperture.